The invention relates to modular plastic conveyor belts and, more particularly, to hinge pins for connecting consecutive modular belt rows together, wherein the hinge pins are made of an oriented polymer material.
Because they do not corrode and are easy to maintain, modular plastic conveyor belts are widely used in many industries to convey articles. These belts, or chains, are typically constructed of a series of rows of one or more belt modules connected end to end into an endless conveyor belt. Each row includes spaced apart hinge elements at each end with apertures formed through them. The hinge elements along one end of a row are interleaved with the hinge elements along an end of an adjacent row. The apertures of the interleaved hinge elements are aligned and form a passageway. A hinge pin inserted in the passageway serves to connect the rows together and to allow them to pivot about the axis of the pin so that the belt can backflex or articulate about a drive sprocket. Hinge pins are typically molded or extruded of a thermoplastic material, such as polypropylene, polyethylene, nylon, acetal, or composite polymers.
As the belt moves, belt tension is borne not only by the modules but also by the hinge pins. Each row of belt modules pulls the trailing rows. This belt pull, or tension, is transmitted row to row through the hinge pins. With each row of interleaved hinge elements pulling in opposite directions, the hinge pin can, over time, deform, assuming a corrugated, or stepped, shape resembling a cam shaft. Cam-shafting of hinge pins causes problems.
One problem caused by cam-shafted hinge pins is scrubbing of the hinge pin against the walls of the apertures in the hinge elements. Because a cam-shafted hinge pin has a preferred orientation relative to its confining hinge elements, scrubbing and the consequent wear are concentrated on the same portions of the hinge pin's outer surface. This tends to accelerate the wear of the hinge pin.
A second problem caused by cam-shafted hinge pins is elongation of the belt. A cam-shafted hinge pin connecting two adjacent rows of belt modules allows the apertures of one row to move out of alignment with those of the adjacent row, effectively lengthening the pitch of the row and stretching the belt's overall length. Wallowing out of the apertures by scrubbing against the hinge pin adds to the belt-stretching problem. The increased length means more sag in the belt on the returnway, for which space must be allocated. The longer pitch of the belt can adversely affect engagement of the belt with fixed-pitch drive and idler sprockets. Poor sprocket engagement makes for a less uniform conveying speed and a bumpier ride. Accelerated wear of, and even damage to, the belt drive surfaces and the sprockets are other results of poor sprocket engagement. Conditions such as heavy loading and high temperatures can exacerbate these problems caused by cam-shafted hinge pins.
One way these problems have been addressed is through the use of larger diameter hinge pins for increased strength and less susceptibility to deformation. Another approach is to change the geometry of the modules to compensate for weak hinge pins. But these approaches often represent compromises that adversely affect other performance characteristics. For example, larger hinge pins require larger hinge-element apertures, which means that, with more material removed, the hinge element is inherently weaker.
Yet another approach is the use of stainless steel hinge pins, which are inherently stronger and stiffer and unlikely to deform as much as conventional plastic hinge pins. But steel hinge pins are heavy. The added weight increases the tensile load on the belt modules and may require larger drive motors. Furthermore, special pin retention schemes are often necessary to retain stainless steel hinge pins in belts.
Thus, there is a need for a modular plastic conveyor belt that does not stretch over time, especially in heavy-load applications.